1. Field of the Invention
The present invention relates to an apparatus and method for power allocation in a mobile communication system. More particularly, the present invention relates to an apparatus and method in which power is effectively allocated to a Forward Link (FL) control channel in a mobile communication system.
2. Description of the Related Art
In general, in order for mobile communication systems to provide power control, handoff; transmission of a Signal to Interference and Noise Ratio (SINR), and transmission of Automatic Repeat reQuest (ARQ) information, a control channel is used in a Forward Link (FL) and a Reverse Link (RL).
FIG. 1 illustrates a signal flow between an Access Terminal (AT) operating in a soft-handoff mode and a Base Transceiver Station (BTS).
Referring to FIG. 1, an AT receives FL packets and FL control channels from two BTSs, and transmits RL packets and RL control channels to the BTSs. A Base Station Controller (BSC) receives from the two BTSs the signals provided from the AT, combines these signals, and transmits FL data packets and FL control signals to the two BTSs.
In general, when a plurality of ATs exist in a service area of a BTS, the BTS can use Time Division Multiplexing (TDM) or Code Division Multiplexing (CDM) in order to transmit a plurality of control channels corresponding to the respective ATs. In the following description, it is assumed that the control channels are transmitted by performing CDM.
FIG. 2 illustrates an FL slot format in a Code Division Multiple Access (CDMA) mobile communication system, in particular, in a CDMA 1×EVolution-Data Only (1×EV-DO) system. Referring to FIG. 2, one slot is divided into two half slots, each of which transmit data, a Media Access Control (MAC) channel, and a pilot channel. An FL control channel is transmitted through the MAC channel.
FIG. 3 illustrates a control channel format in a CDMA mobile communication system. That is, when a control channel is transmitted to four ATs through the MAC channel of FIG. 2, a signal transmitted in the slot unit has the format shown in FIG. 3.
Referring to FIG. 3, an FL control channel includes a Reverse Activity (RA) channel, a Data Rate Control Lock (DRCLock) channel, a Reverse Power Control (RPC) channel, and an ARQ channel. The RA channel is commonly transmitted to all ATs. The DRCLock channel, the RPC channel, and the ARQ channel are independently assigned to each AT. Each FL control channel is distinguished using an orthogonal code. That is, orthogonality is mutually maintained since the control channels transmitted in the same slot are spread by use of different codes (e.g., Walsh code).
The DRCLock channel and the RPC channel are concurrently transmitted in one slot. The ARQ channel is time-division multiplexed with the DRCLock/RPC channel and is transmitted for three slots. Transmission time points of the DRCLock/RPC channel and the ARQ channel are determined by a frame offset of an AT, as shown by Equation (1) below.If (T−FrameOffsetk) mod 4=3, transmit DRCLock/RPC channel Else transmit ARQ channel   (1)
Here, T denotes a time in the slot unit, FrameOffsetk denotes a frame offset of a kth AT, and “x mod y” denotes a modular operation for obtaining a remainder of division of x by y. In Equation (1) above, since the transmission time point of the DRCLock/RPC channel for the kth AT is determined by a value of (FrameOffsetk mod 4), ATs can be classified into four groups as shown in FIG. 3 according to their frame offsets.
By considering that each AT has a different transmission time point, a BTS verifies a control channel to be transmitted to each AT for each slot, and divides the total transmission power of a MAC channel to ATs.
A brief description of a conventional algorithm for MAC channel power allocation is now provided.
According to the conventional algorithm for MAC channel power allocation, total MAC channel power is divided in advance for an RA channel, a DRCLock channel, an RPC channel, and an ARQ channel, and then the divided power is used to allocate power to a corresponding control channel of each AT. For example, assume that the total MAC channel power is allocated so that 10% is used by the RA channel, 30% is used by the DRCLock channel, 30% is used by the RPC channel, and 30% is used by the ARQ channel. Then, 60% of the total power is allocated to a group of ATs for transmitting the DRCLock/RPC channel, and 30% of the total power is allocated to a group of ATs for transmitting the ARQ channel.
As such, when power allocated to each channel is divided in advance, if the total MAC channel power is less than a sum of the power required by an AT, some channels may experience performance degradation. For example, if a frame offset of the AT mostly appears in one of the four groups of FIG. 3, and thus the number of DRCLock/RPC channels is greater than a fraction of power allocated to the DRCLock/RPC channel, then reception throughput of the DRCLock/RPC channel deteriorates. Likewise, if the number of ARQ channels is greater than a fraction of power allocated to the ARQ channel, reception throughput of the ARQ channel deteriorates.
Accordingly, when using conventional algorithms in which the total MAC channel power is divided in advance for the DRCLock channel, the RPC channel, and the ARQ channel before power allocation is performed, reception throughput may deteriorate in some channels if power is inaccurately divided for each channel. Therefore, there is a need for a method in which total power allocated to an FL MAC channel can be effectively allocated to each control channel.